Solar cell modules generally have a photosensitive semiconductor layer (hereinafter may be referred to as a solar cell unit) having a transparent cover for protecting against external influences. Such solar cell units are often disposed between a glass plate and a hard cover plate such as glass or a back sheet, and fixed with a sealing material adhesive to glass.
Since solar cell units are extremely fragile, cross-linkable sealing materials composed of an ethylene-vinyl acetate copolymer (hereinafter may be referred to as EVA) or a curable casting resin as a base have been used as described, for example, in Japanese Patent Application Laid-Open No. 1983-023870 (Patent Literature 1) or Japanese Patent Application Laid-Open No. 1994-177412 (Patent Literature 2). In an uncured state, the viscosity of these sealing materials may be adjusted to be low so that the solar cell unit is covered with the material. After covering the solar cell unit in that way, a cross-linking reaction by a curing agent or a cross-linking agent allows the sealing material to possess a certain level of mechanical strength. In solar cell modules using EVA, however, there is a problem of corrosion of metal components caused by acetic acid formed by hydrolysis or thermal decomposition of EVA. Moreover, due to the low viscosity before curing, EVA also has a problem of flowing from the edge of glass to contaminate laminators and glass surfaces. As for the curable casting resin, embedding of solar cell units and control of curing is extremely difficult, and the resin often causes problems such as generation of bubbles or peeling several years after the manufacture of the solar cell module. Therefore, few curable casting resins have been used as a sealing material for solar cells.
Also, as described in Japanese Patent Application Laid-Open No. 2006-013505 (Patent Literature 3), etc., a film composed of thermoplastic polyvinyl butyral (hereinafter may be referred to as PVB) resin as a base is used as a sealing material. Since the content of acetic acid residue which forms an acid component is low in PVB, PVB has an advantage that corrosion of metal components is less likely to occur as compared to EVA. Further, since PVB is thermoplastic, it is highly viscous at a flow-starting temperature, and thus the resin is less likely to flow from the edge of glass to contaminate devices or glass surfaces. Moreover, from the standpoint of mechanical strength, film containing PVB resin has excellent adhesiveness to glass and penetration resistance, and thus is useful as an intermediate film for car windshields and architectural laminated safety glass.
Generally, when using polyvinyl acetal such as polyvinyl butyral as a sealing material for a solar cell or an intermediate film for laminated glass, a plasticizer is added. Examples of plasticizers used include an acrylic polymer having a number average molecular weight of 1000 or more and a weight average molecular weight of 2000 or more, which is described, for example, in Japanese Patent Application Laid-Open No. 2004-250254 (Patent Literature 4). Also, Patent Literature 4 describes that since laminated glass used for car, aircraft or building windows is generally produced by a vacuum bag or vacuum ring process performed at a relatively low degree of vacuum, or by a nip roll process or a heat press process at ordinary pressure, low molecular weight volatile substances cause some problems. Further, International Publication No. WO2009/135928 (Patent Literature 5) discloses a plasticized polyvinyl acetal film containing 10 to 40% by mass of cyclohexane dicarboxylic acid ester (e.g., cyclohexane dicarboxylic acid diisononyl ester) and having a Tg of 74° C. or less. The plasticizer has excellent compatibility with polyvinyl acetal film and thus can provide a film capable of maintaining flexibility even in cold conditions.
Methods using a vacuum laminator performed under high vacuum conditions are described in Japanese Patent Application Laid-Open No. 1998-214987 (Patent Literature 6) and Japanese Patent Application Laid-Open No. 1986-069179 (Patent Literature 7).
For polyvinyl acetal resin with a low volatile component content, Japanese Patent Application Laid-Open No. 2001-072445 (Patent Literature 8), for example, describes an intermediate film for laminated glass in which the total content of volatile substances after being left in an atmosphere at 100° C. for 1 hour is 30 ppm or less. Also, Japanese Patent Application Laid-Open No. 2006-047974 (Patent Literature 9) describes a polyvinyl acetal resin for a heat developable photosensitive material having a 2-ethyl-2-hexenal content of 60 ppm or less.